1. Field of the Invention
The present invention relates to a low melting point glass suitable for covering for insulation of transparent electrodes of e.g. ITO (indium oxide doped with tin) or tin oxide (inclusive of tin oxide doped with e.g. fluorine or antimony), and a plasma display device.
2. Discussion of Background
In recent years, a thin flat plate type color display device has attracted an attention. In such a display device, it is necessary to form an electrode for each pixel in order to control the display state in the pixel for forming an image. In order to prevent deterioration of the image quality, transparent electrodes are used as such electrodes. As the transparent electrodes, thin films of ITO or tin oxide formed on a glass substrate, are commonly employed.
Especially, transparent electrodes which are formed on the surface of a glass substrate to be used as a display panel of the above display device, are formed into fine lines to realize fine images. In order to control the respective pixels independently, it is necessary to secure insulation among such finely formed transparent electrodes. However, if moisture is present on the surface of the glass substrate, or if an alkali component is present in the glass substrate, it may happen that an electrical current flows to some extent via the surface of this glass substrate. To prevent such a current, it is effective to form an insulating layer between the transparent electrodes. Further, in order to prevent deterioration of the image quality by the insulating layer formed between the transparent electrodes, such an insulating layer is preferably transparent.
Various materials are known as an insulating material for forming such an insulating layer. Among them, a glass material is widely employed which is a transparent and highly reliable insulating material.
For a front substrate of a plasma display device which is recently expected as a large size flat color display device (typically a display device having cells defined and formed by a front substrate used as a display surface, a rear substrate and barrier ribs and designed to generate plasma discharge in the cells to form an image, which will be referred to as PDP), a glass-covering layer excellent in plasma durability is essential to protect the above transparent electrodes from plasma.
Further, also with respect to the rear substrate of PDP, electrodes formed thereon will be covered by a glass covering layer. For this glass covering layer, high light reflectance is required in many cases to effectively use a light generated by plasma from a phosphor formed on barrier ribs, as a display light, and a light-shielding property may sometimes be required to prevent leakage of the above light from the rear substrate.
Such a glass covering layer may be formed by a method of forming it in vacuum by e.g. sputtering. However, it has been common to employ a method wherein a low melting point glass powder is made into a paste, which is coated and fired on a glass substrate.
Further, electrodes are required to be exposed at the wiring portions connected to a driving circuit. Heretofore, it has been common to cover the wiring portions connected to a driving circuit by means of a ZnO-containing paste excellent in solubility in an acid, which is separate from the paste to be used for the glass covering layer and to finally remove the covering by means of an acidic solution such as dilute nitric acid, as disclosed, for example, in JP-A-6-267424.
To suppress the reaction between transparent electrodes, and a glass covering layer at the time of forming the glass covering layer on the transparent electrodes by firing a low melting point glass powder, JP-A-7-105855 discloses a glass covering layer having a double layer structure which comprises a high softening point glass layer as the lower layer and a low softening point glass layer as the upper layer. However, there has been a problem that the number of process steps increases as compared with a glass covering layer having a single layer structure.
Further, a conventional glass covering layer has had a high dielectric constant. Therefore, the capacitance of cells tends to be large, and the discharge current per plasma emission tends to be large, whereby there has been a problem that the electric power consumption of PDP is high. JP-A-8-77930 discloses a glass covering layer having a relative dielectric constant of at most 8, but a protective layer is formed between the electrodes and the glass covering layer in order to prevent the reaction between the electrodes and the glass covering layer or to prevent diffusion of the alkali cation in the glass covering layer to the electrodes. Hereinafter, a relative dielectric constant will be referred to simply as a dielectric constant.
Further, with respect to the exposure of electrodes at the wiring portions connected to a driving circuit, JP-A-10-316451 discloses use of a glass covering layer readily soluble in an acid. It is thereby possible to avoid an increase of process steps or an increase in the number of covering materials, as it is unnecessary to apply a separate covering at the wiring portions connected to a driving circuit. However, the dielectric constant of the glass covering layer disclosed in this publication is as high as 15, whereby the above-mentioned problem relating to the dielectric constant still remains.
It is an object of the present invention to provide a low melting point glass for covering electrodes, whereby a glass covering layer can be made to have a single layer structure, the dielectric constant is low and no separate covering is required at the wiring portions connected to a driving circuit, a glass ceramic composition for covering electrodes whereby no separate covering is required at wiring portions connected to a driving circuit, and a plasma display device using the above low melting point glass for covering electrodes and/or the glass ceramic composition for covering electrodes.
The present invention provides a low melting point glass for covering electrodes, consisting, as represented by mass percentage based on the following oxides, essentially of:
Further, the present invention provides a glass ceramic composition for covering electrodes, which comprises 100 parts by mass of a powder of the above-mentioned low melting point glass for covering electrodes and from 0.5 to 40 parts by mass of a powder of an inorganic pigment.
Still further, the present invention provides a plasma display device having a front substrate, wherein transparent electrodes formed on a glass substrate constituting the front substrate, are covered by the above-mentioned low melting point glass for covering electrodes.
Furthermore, the present invention provides a plasma display device having a rear substrate, wherein electrodes formed on a glass substrate constituting the rear substrate, are covered by a fired product of the above-mentioned glass ceramic composition for covering electrodes.